Can't be done in Sigil alone, so, we need to do it by using placeholder tags, then ripping open the html and substituting the video or audio tags.

Is this what we need to do?
sorry if my question was pointless.

I think what Hitch is saying: Put a Picture of the screen shape size your
video will occupy (aplacemat.jpg. could even be a screen shot of the title),and note the filename that contains it.

AFTER your EPUB has passed Flightcrew and been FINAL saved.
Unpack the Epub and replace the place holder code, image in the file you noted, and re-pack the EPUB.

Maybe we can get John to loosen up the Pure EPUB spec enforcement, and allow 'Non-Spec items' to be included after answering a few Nag boxes ("I authorize non-compliant inclusion")
Flightcrew only Yellow Flags extra files , so why not treat a Video as 'extra'?